Gram-Positive Cocci Overview

Gram-Positive Cocci

• Gram-positive cocci are spherical bacteria arranged in characteristic patterns
• Gram-positive cocci can be divided into staphylococci and streptococci
• Staphylococci form grape-like clusters while streptococci form chains

Staphylococci

• General features: spherical, Gram-positive, arranged in grape-like clusters, catalase-positive, non-motile, facultative anaerobic bacteria
• Medically important species include Staphylococcus aureus (most important), Staphylococcus epidermidis and Staphylococcus saprophyticus
• Classification based on coagulase enzyme: coagulase-positive (produce coagulase) and coagulase negative (do not produce coagulase)
  • Staphylococcus aureus is the example of coagulase-positive
  • Staphylococcus epidermidis and Staphylococcus saprophyticus are examples of coagulase-negative species
• Coagulase: enzyme inducing plasma coagulation by activating prothrombin to thrombin and then fibrin
• important virulence factors: Teichoic acid, Collagen-binding proteins (in attachment), and different exotoxins (proteases like collagenase and elastase, lipases, hyaluronidase, Pore-forming toxins like PV Leukocidin, Toxic Shock Syndrome Toxin (TSST), Exfoliative toxin(scalded skin syndrome) , Enterotoxin B)

Staphylococcal Infections

• Local skin infections (tissue destruction and pus formation): folliculitis, boils, wound infections, furuncles, carbuncles
• Systemic infections (reach the blood stream): septicemia, endocarditis, osteomyelitis, meningitis, pneumonia, septic shock
• Septic shock can be caused by Teichoic acid or TSST.

Staphylococcus aureus

• Epidemiology/Transmission: 30-50% of people carry in anterior nares, 5% in vaginas, spreading possible via hands, skin, throat and other surfaces
• MRSA (Methicillin Resistant Staphylococcus aureus) common, VRSA appeared
• important virulence factors: Teichoic acid, Collagen-binding proteins, Exotoxins and Exfoliative Toxin: involved in scalded skin syndrome and causes tissue damage, and Enterotoxin B involved in food poisoning.

Other diseases caused by S. aureus

• Non-pyogenic skin conditions: cellulitis and impetigo
• Scalded skin syndrome(SST): epidermolytic toxins (exfoliative toxins) cleave desmogleins in superficial skin layers causing massive skin exfoliation; fever, large areas of skin slough, secondary infections, electrolyte imbalances can occur

Staphylococcal Food Poisoning

• Ingestion of food heavily contaminated with S. aureus strains containing enterotoxin B can result in self-limiting vomiting and watery diarrhea.
• Enterotoxin B: a super-antigen that induces cytokine production in mesenteric lymph nodes after food ingestion
• Cytokines stimulate vomiting center in the brain

Staphylococcus epidermidis and Staphylococcus saprophyticus

• Both coagulase-negative staphylococci
• S. epidermidis is part of the normal human skin flora (90% of normal skin microbiota) found on skin and mucous membranes
• It commonly infects intravenous catheters and prosthetic implants (e.g. prosthetic heart valves, vascular grafts, prosthetic joints (arthritis or osteomyelitis)
• It can cause sepsis in neonates and peritonitis in patients with renal failure on peritoneal dialysis
• S. saprophyticus causes urinary tract infections, especially in sexually active young women

Streptococcus

• Spherical, Gram-positive cocci that form chains
• Catalase-negative
• Are non-motile
• Facultative anaerobes
• Classified based on hemolytic activity: Alpha-hemolytic, Beta-hemolytic, Gamma-hemolytic
  • Alpha-hemolytic: green zone around colonies due to hemoglobin reduction to methemoglobin
  • Beta-hemolytic: clear zone around colonies due to complete lysis of red blood cells
  • Gamma-hemolytic: no change around colonies
• Classification based on Lancefield Grouping: important CHO antigen of the cell wall (e.g. C carbohydrate)

Streptococcus pyogenes (Beta-hemolytic/Group A)

• Key pathogen
• Commonly found in skin and throat
• Important virulence factors: M protein (anti-phagocytic), capsule (anti-phagocytic, surface disguise), Teichoic acid(can cause toxic shock), Pore-forming toxins (hemolysins like Streptolysin), Pyogenic exotoxin A, (super-antigen, causing toxic shock)
• Infections: Pharyngitis (strep throat), scarlet fever (rash, strawberry tongue), Otitis media, bronchitits, pneumonia (bacteremia, septic shock), meningitis, non-pyogenic skin infections (cellulitis and impetigo), toxic shock (systemic disease), Streptococcal gangrene (necrotizing fasciitis), Erysipelas, Puerperal sepsis (childbirth fever)
• Post-Streptococcal diseases: Acute glomerulonephritis, acute rheumatic fever (immune reaction against streptococcal M protein), subcutaneous nodules, Sydenham’s chorea

Post-Streptococcal diseases

• Acute glomerulonephritis: inflammatory condition affecting glomeruli (kidneys) due to cross-reacting antibodies binding to human antigens
• Acute rheumatic fever: inflammation of heart valves caused by immune response to streptococcal M protein and other antigens causing heart valve damage

Other Streptococcus

• Viridans streptococci: commonly found in the oropharynx
• Streptococcus agalactiae: found in vagina and colon
• Enterococci and anaerobic streptococci: found in the colon